Light emitting diodes (LEDs) are attractive candidates for replacing conventional light sources such as incandescent lamps and fluorescent light sources. The LEDs have higher light conversion efficiencies and longer lifetimes than the conventional sources. LEDs are particularly attractive candidates for light sources that are utilized in back-lit displays based on thin extended light sources that are placed behind a transparent object that is illuminated. Such displays are routinely used in handheld devices such as cellular telephones, laptop computers, and PDAs. The illumination system typically utilizes some form of light box or light pipe behind a transparency that could be a fixed scene or a liquid crystal display (LCD) panel. Light is injected into this light box at the periphery of the light box. The surface of the light box opposite to the surface that is adjacent to the transparency has some form of scattering center that scatters the light so that the back surface of the LCD is uniformly illuminated.
The thickness of the light source is limited by the thickness of the light box. The thickness of the display is particularly important in displays used for laptop computers and handheld devices such as PDAs and cellular telephones, as the display thickness limits the overall thickness of the device. Some of these portable devices require light boxes that are less than 1 mm thick. As the thickness of the light box is reduced, the efficient injection of light into the light box from the sides in a manner that assures uniform illumination and high efficiency becomes more difficult.